The channels are listed / edited in a grid.

The columns have the following functions:

Number
Channel number. This is not used by Tvheadend internally, but rather intended to be used by HTSP clients for mapping to remote control buttons, presentation order, etc
Name
Name of the channel as presented in EPG and external interfaces. Changing name of a channel does not interfere with scheduled recordings etc.
Play
Play the channel using the VLC plugin (if available) else it will display a direct link that can be used to open in preferred media player.
EPG Grab Source
Name of the Internet-based EPG provider (typically XMLTV) channel that should be used to update this channels EPG info. By default Tvheadend tries to match the name itself, but sometimes it might not match correctly in which case you can do the mapping manually. When using XMLTV, a newly-mapped channel will not appear in the EPG until the next run of the grabber, or service restart.

Over the air (OTA) sources do NOT need to be mapped in this way, the linking is implicitly tied since they are all part of the DVB system.

Tags
Each channel can be bound to a zero, one or many tags. The tags are used for multiple things and it's up to you how you wish to use and name them. Tags can be used to aid searching in the EPG if you have many channels. The tags are also presented in a Media player.
Icon
An URL pointing to an image representing the channel. The icon URL will be set automatically when importing data from XMLTV. This field allows the user to edit it manually.
DVR Pre-Start
Allows the user to specify an amount of extra time that should be prepended to a recording scheduled on this channel. In other words, if you type 5 here for a recording that is scheduled for 20:00 will start to record at 19:55. This setting overrides the Extra time before recordings DVR setting.
DVR Post-End
Similar to DVR Pre-Start this allows the user to add an extra amount of time that should be appended to a recording. In other words, if you type 5 here for a recording that is scheduled to end 20:30 will stop to record at 20:35. This setting overrides the Extra time after recordings DVR setting.
Merge button
If you have two channels that are actually the same you can press the merge button on one of the channels to merge that into another channel. After the merge the channel you pressed this function on will be removed and all services connected to it will be added to the target channel.